In the refrigeration and air conditioning industry it has become a wide spread practice to utilize all aluminum heat exchanger coils. These coils are used within both the condensing units and the evaporating units of many refrigeration and air conditioning applications. In particular, aluminum is used because it is less costly than copper and more available. A typical problem of an aluminum heat exchanger coil is that once the coil suffers a fracture or other damage occurs within a section thereof it has heretofore been difficult to substitute a replacement section for the faulty section in the field. With copper coils, the section to be replaced was merely unsoldered and a new one soldered in place. However, all aluminum coils have been typically assembled by joining certain parts using a flux solder process or an ultrasonic dip solder process wherein special equipment is utilized to join the two aluminum parts. It is highly impracticable to make field repairs utilizing either the flux solder method or the ultrasonic dip soldering process.
Adhesives have been suggested for utilization in combining the various components of all aluminum heat exchanger coils. In U.S. Pat. No. 3,498,866 issued to Kilbane and assigned to Minnesota Mining and Manufacturing Company, there is disclosed a method of making a tube joint by applying an adhesive collar onto the end surface of the tubing section. Other patents have been issued dealing with adhesive bonding of the coils. In particular U.S. Pat. No. 3,636,186 issued to R. A. Sturley, deals with the factory bonding of tube joints by an accurate metering of a predetermined quantity of thermosetting adhesive to a particular location of a joint as does U.S. Pat. No. 3,796,783 issued to J. E. Greever, a co-inventor herein. Both of these patents require a treatment of some nature to the adhesive, either providing a particular collar arrangement or metering the adhesive into the joint. Neither of these is particularly suitable for field use in repairing the faulty section of the coil.
It has been found that when field repairs are made to all aluminum coils that the faulty section may be unsoldered from the coil using conventional unsoldering processes. The application of heat to the joint to be unsoldered is sufficient to melt the solder utilized by either the flux process or the ultrasonic dip soldering process. When the section is removed from the coil, roughened surfaces are left which are suitable for bonding by an adhesive. However, the replacement part supplied by the factory has been exposed to the atmosphere and consequently has a surface layer of aluminum oxide formed thereon. By applying adhesive directly to these aluminum oxides the strength of the joint is greatly reduced since the bonding strength of the aluminum oxide is significantly less than that of the underlying aluminum. The invention described herein provides for roughening the surface of the replacement part to increase adhesive adherence and thereafter applying a conversion coating to the surface such that the aluminum oxides are replaced with the conversion coating and the formation of additional aluminum oxide is prevented and simultaneously a strong surface adapted to be bonded by an adhesive is provided.